This invention relates in general to a navigational aid computer for plotting courses on marine charts.
As is well known, conventional marine charts include a rectangular grid representing meridians of longitude and parallels of latitude with the meridians of longitude oriented to the true north direction. However, a marine navigator usually employs magnetic compass headings as the chief guide to maintaining the vessel on course. Since the variation between true north and magnetic north varies as a function of latitude, the variation for any particular area is imprinted on the chart. This is done in the form of a "compass rose" which includes two concentric circular scales divided into 360.degree. marks, the outer one being oriented to true north at zero degrees with the inner one being rotated an amount equal to the magnetic variation.
The usual technique for plotting a course includes laying out a line of course on the chart and using a parallel rule device to translate this course until it overlies the compass rose so that the magnetic direction of the course, as well as the true direction of the course, can be determined. The procedure is somewhat inconvenient since it requires the repetitive translation of the course by means of the parallel rule until the compass rose is overlaid. This operation is particularly awkward when a small boat is operated under adverse conditions of weather and sea. Various devices, have, in the past, been proposed and employed for carrying out this operation more conveniently. One such device includes a pair of transparent disks fastened at a common axis to permit rotation of one relative to the other, with each of the disks being divided into a 360.degree. scale and further including a narrow straightline piece, also pivoted at the common axis. In operation, this device is placed overlying the plotted course with the zero degree mark on one of the disks designated as true north being aligned with true north on the chart and the other disk being rotated such that its zero degree marker, representing magnetic north, is rotated the correct number of degrees representing the magnetic variation for that chart, as displayed on the compass rose for that chart. The narrow straightline piece is then aligned along the direction of course and an axial center line along the narrow straightline piece, provides a reading from one disk of the magnetic heading and from the other of the true heading.
To perform this operation, it is necessary to have the axis of the navigational aid device superimposed on one of the meridians of longitude, in order to align the device with true north.